Add filterSerializedResponseHeaders function (#6569)

* refactor

* add filterSerializedResponseHeaders option - closes #1971

* Update .changeset/silent-cycles-reply.md

Co-authored-by: Conduitry <git@chor.date>

* Update documentation/docs/06-hooks.md

Co-authored-by: Conduitry <git@chor.date>

* throw error if excluded header is read during SSR

* argh

* fix

Co-authored-by: Conduitry <git@chor.date>

.changeset/silent-cycles-reply.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+[breaking] exclude headers from serialized responses by default, add `filterSerializedResponseHeaders` `resolve` option

.gitignore

@@ -4,6 +4,7 @@ dist
 test-results/
 package-lock.json
 yarn.lock
+vite.config.js.timestamp-*
 /packages/create-svelte/template/CHANGELOG.md
 /packages/package/test/**/package
 /documentation/types.js
@@ -16,3 +17,4 @@ yarn.lock
 .turbo
 .vercel
 .test-tmp
+

documentation/docs/05-load.md

@@ -143,7 +143,7 @@ export async function load({ depends }) {
 - it can be used to make credentialed requests on the server, as it inherits the `cookie` and `authorization` headers for the page request
 - it can make relative requests on the server (ordinarily, `fetch` requires a URL with an origin when used in a server context)
 - internal requests (e.g. for `+server.js` routes) go direct to the handler function when running on the server, without the overhead of an HTTP call
-- during server-side rendering, the response will be captured and inlined into the rendered HTML
+- during server-side rendering, the response will be captured and inlined into the rendered HTML. Note that headers will _not_ be serialized, unless explicitly included via [`filterSerializedResponseHeaders`](/docs/hooks#handle)
 - during hydration, the response will be read from the HTML, guaranteeing consistency and preventing an additional network request
 
 > Cookies will only be passed through if the target host is the same as the SvelteKit application or a more specific subdomain of it.

documentation/docs/06-hooks.md

@@ -64,13 +64,15 @@ You can add call multiple `handle` functions with [the `sequence` helper functio
 `resolve` also supports a second, optional parameter that gives you more control over how the response will be rendered. That parameter is an object that can have the following fields:
 
 - `transformPageChunk(opts: { html: string, done: boolean }): MaybePromise<string | undefined>` — applies custom transforms to HTML. If `done` is true, it's the final chunk. Chunks are not guaranteed to be well-formed HTML (they could include an element's opening tag but not its closing tag, for example) but they will always be split at sensible boundaries such as `%sveltekit.head%` or layout/page components.
+- `filterSerializedResponseHeaders(name: string, value: string): boolean` — determines which headers should be included in serialized responses when a `load` function loads a resource with `fetch`. By default, none will be included.
 
 ```js
 /// file: src/hooks.js
 /** @type {import('@sveltejs/kit').Handle} */
 export async function handle({ event, resolve }) {
 	const response = await resolve(event, {
-		transformPageChunk: ({ html }) => html.replace('old', 'new')
+		transformPageChunk: ({ html }) => html.replace('old', 'new'),
+		filterSerializedResponseHeaders: (name) => name.startsWith('x-')
 	});
 
 	return response;

packages/kit/src/core/prerender/prerender.js

@@ -237,10 +237,11 @@ export async function prerender() {
 			const encoded_dependency_path = new URL(dependency_path, 'http://localhost').pathname;
 			const decoded_dependency_path = decodeURI(encoded_dependency_path);
 
-			const prerender = result.response.headers.get('x-sveltekit-prerender');
+			const headers = Object.fromEntries(result.response.headers);
 
+			const prerender = headers['x-sveltekit-prerender'];
 			if (prerender) {
-				const route_id = /** @type {string} */ (result.response.headers.get('x-sveltekit-routeid'));
+				const route_id = headers['x-sveltekit-routeid'];
 				const existing_value = prerender_map.get(route_id);
 				if (existing_value !== 'auto') {
 					prerender_map.set(route_id, prerender === 'true' ? true : 'auto');
@@ -259,7 +260,10 @@ export async function prerender() {
 			);
 		}
 
-		if (config.prerender.crawl && response.headers.get('content-type') === 'text/html') {
+		// avoid triggering `filterSerializeResponseHeaders` guard
+		const headers = Object.fromEntries(response.headers);
+
+		if (config.prerender.crawl && headers['content-type'] === 'text/html') {
 			for (const href of crawl(body.toString())) {
 				if (href.startsWith('data:') || href.startsWith('#')) continue;
 
@@ -288,7 +292,9 @@ export async function prerender() {
 	 */
 	function save(category, response, body, decoded, encoded, referrer, referenceType) {
 		const response_type = Math.floor(response.status / 100);
-		const type = /** @type {string} */ (response.headers.get('content-type'));
+		const headers = Object.fromEntries(response.headers);
+
+		const type = headers['content-type'];
 		const is_html = response_type === REDIRECT || type === 'text/html';
 
 		const file = output_filename(decoded, is_html);
@@ -297,15 +303,15 @@ export async function prerender() {
 		if (written.has(file)) return;
 
 		if (response_type === REDIRECT) {
-			const location = response.headers.get('location');
+			const location = headers['location'];
 
 			if (location) {
 				const resolved = resolve(encoded, location);
 				if (is_root_relative(resolved)) {
 					enqueue(decoded, decodeURI(resolved), resolved);
 				}
 
-				if (!response.headers.get('x-sveltekit-normalize')) {
+				if (!headers['x-sveltekit-normalize']) {
 					mkdirp(dirname(dest));
 
 					log.warn(`${response.status} ${decoded} -> ${location}`);

packages/kit/src/runtime/server/index.js

@@ -14,6 +14,8 @@ import { DATA_SUFFIX } from '../../constants.js';
 /** @param {{ html: string }} opts */
 const default_transform = ({ html }) => html;
 
+const default_filter = () => false;
+
 /** @type {import('types').Respond} */
 export async function respond(request, options, state) {
 	let url = new URL(request.url);
@@ -201,7 +203,8 @@ export async function respond(request, options, state) {
 
 	/** @type {import('types').RequiredResolveOptions} */
 	let resolve_opts = {
-		transformPageChunk: default_transform
+		transformPageChunk: default_transform,
+		filterSerializedResponseHeaders: default_filter
 	};
 
 	/**
@@ -226,7 +229,8 @@ export async function respond(request, options, state) {
 				}
 
 				resolve_opts = {
-					transformPageChunk: opts.transformPageChunk || default_transform
+					transformPageChunk: opts.transformPageChunk || default_transform,
+					filterSerializedResponseHeaders: opts.filterSerializedResponseHeaders || default_filter
 				};
 			}
 

packages/kit/src/runtime/server/page/fetch.js

@@ -10,9 +10,10 @@ import { domain_matches, path_matches } from './cookie.js';
  *   state: import('types').SSRState;
  *   route: import('types').SSRRoute | import('types').SSRErrorPage;
  *   prerender_default?: import('types').PrerenderOption;
+ *   resolve_opts: import('types').RequiredResolveOptions;
  * }} opts
  */
-export function create_fetch({ event, options, state, route, prerender_default }) {
+export function create_fetch({ event, options, state, route, prerender_default, resolve_opts }) {
 	/** @type {import('./types').Fetched[]} */
 	const fetched = [];
 
@@ -189,16 +190,6 @@ export function create_fetch({ event, options, state, route, prerender_default }
 				async function text() {
 					const body = await response.text();
 
-					// TODO just pass `response.headers`, for processing inside `serialize_data`
-					/** @type {import('types').ResponseHeaders} */
-					const headers = {};
-					for (const [key, value] of response.headers) {
-						// TODO skip others besides set-cookie and etag?
-						if (key !== 'set-cookie' && key !== 'etag') {
-							headers[key] = value;
-						}
-					}
-
 					if (!body || typeof body === 'string') {
 						const status_number = Number(response.status);
 						if (isNaN(status_number)) {
@@ -214,14 +205,27 @@ export function create_fetch({ event, options, state, route, prerender_default }
 								? request.url.slice(event.url.origin.length)
 								: request.url,
 							method: request.method,
-							body: /** @type {string | undefined} */ (request_body),
-							response: {
-								status: status_number,
-								statusText: response.statusText,
-								headers,
-								body
-							}
+							request_body: /** @type {string | undefined} */ (request_body),
+							response_body: body,
+							response: response
 						});
+
+						// ensure that excluded headers can't be read
+						const get = response.headers.get;
+						response.headers.get = (key) => {
+							const lower = key.toLowerCase();
+							const value = get.call(response.headers, lower);
+							if (value && !lower.startsWith('x-sveltekit-')) {
+								const included = resolve_opts.filterSerializedResponseHeaders(lower, value);
+								if (!included) {
+									throw new Error(
+										`Failed to get response header "${lower}" — it must be included by the \`filterSerializedResponseHeaders\` option: https://kit.svelte.dev/docs/hooks#handle`
+									);
+								}
+							}
+
+							return value;
+						};
 					}
 
 					if (dependency) {

packages/kit/src/runtime/server/page/index.js

@@ -131,7 +131,8 @@ export async function render_page(event, route, page, options, state, resolve_op
 			options,
 			state,
 			route,
-			prerender_default: should_prerender
+			prerender_default: should_prerender,
+			resolve_opts
 		});
 
 		if (get_option(nodes, 'ssr') === false) {

packages/kit/src/runtime/server/page/render.js

@@ -284,7 +284,11 @@ export async function render_response({
 	}
 
 	if (page_config.ssr && page_config.csr) {
-		body += `\n\t${fetched.map((item) => serialize_data(item, !!state.prerendering)).join('\n\t')}`;
+		body += `\n\t${fetched
+			.map((item) =>
+				serialize_data(item, resolve_opts.filterSerializedResponseHeaders, !!state.prerendering)
+			)
+			.join('\n\t')}`;
 	}
 
 	if (options.service_worker) {
@@ -321,6 +325,7 @@ export async function render_response({
 		})) || '';
 
 	const headers = new Headers({
+		'x-sveltekit-page': 'true',
 		'content-type': 'text/html',
 		etag: `"${hash(html)}"`
 	});

packages/kit/src/runtime/server/page/respond_with_error.js

@@ -25,7 +25,8 @@ export async function respond_with_error({ event, options, state, status, error,
 		event,
 		options,
 		state,
-		route: GENERIC_ERROR
+		route: GENERIC_ERROR,
+		resolve_opts
 	});
 
 	try {

packages/kit/src/runtime/server/page/serialize_data.js

@@ -35,36 +35,51 @@ const pattern = new RegExp(`[${Object.keys(replacements).join('')}]`, 'g');
  * and that the resulting string isn't further modified.
  *
  * @param {import('./types.js').Fetched} fetched
+ * @param {(name: string, value: string) => boolean} filter
  * @param {boolean} [prerendering]
  * @returns {string} The raw HTML of a script element carrying the JSON payload.
  * @example const html = serialize_data('/data.json', null, { foo: 'bar' });
  */
-export function serialize_data(fetched, prerendering = false) {
-	const safe_payload = JSON.stringify(fetched.response).replace(
-		pattern,
-		(match) => replacements[match]
-	);
+export function serialize_data(fetched, filter, prerendering = false) {
+	/** @type {Record<string, string>} */
+	const headers = {};
+
+	let cache_control = null;
+	let age = null;
+
+	for (const [key, value] of fetched.response.headers) {
+		if (filter(key, value)) {
+			headers[key] = value;
+		}
+
+		if (key === 'cache-control') cache_control = value;
+		if (key === 'age') age = value;
+	}
+
+	const payload = {
+		status: fetched.response.status,
+		statusText: fetched.response.statusText,
+		headers,
+		body: fetched.response_body
+	};
+
+	const safe_payload = JSON.stringify(payload).replace(pattern, (match) => replacements[match]);
 
 	const attrs = [
 		'type="application/json"',
 		'data-sveltekit-fetched',
 		`data-url=${escape_html_attr(fetched.url)}`
 	];
 
-	if (fetched.body) {
-		attrs.push(`data-hash=${escape_html_attr(hash(fetched.body))}`);
+	if (fetched.request_body) {
+		attrs.push(`data-hash=${escape_html_attr(hash(fetched.request_body))}`);
 	}
 
-	if (!prerendering && fetched.method === 'GET') {
-		const cache_control = /** @type {string} */ (fetched.response.headers['cache-control']);
-		if (cache_control) {
-			const match = /s-maxage=(\d+)/g.exec(cache_control) ?? /max-age=(\d+)/g.exec(cache_control);
-			if (match) {
-				const age = /** @type {string} */ (fetched.response.headers['age']) ?? '0';
-
-				const ttl = +match[1] - +age;
-				attrs.push(`data-ttl="${ttl}"`);
-			}
+	if (!prerendering && fetched.method === 'GET' && cache_control) {
+		const match = /s-maxage=(\d+)/g.exec(cache_control) ?? /max-age=(\d+)/g.exec(cache_control);
+		if (match) {
+			const ttl = +match[1] - +(age ?? '0');
+			attrs.push(`data-ttl="${ttl}"`);
 		}
 	}